The declining phase of endometrial morphology has been studied in the pseudopregnant rabbit. It is now proposed: (1) to follow and complete the hormonal analysis of the endometrial phases throughout pseudopregnancy with special reference to the role of progesterone and estrogen. The effects on endometrial differentiation of a single small injection of progesterone (1 mg) in the estrous rabbit will be studied; it has been shown that this treatment results in a trend of differentiation leading to ciliation and mucification, unlike the effect of continued progesterone which leads to multinucleated cell formation; (2) studies will be initiated on the pregnant rabbit with special reference to the relationship between multinucleated cells and their interaction with the trophoblast (i.e., its obligatory nature or not), the significance of earlier differentiation of multinucleated cells mesometrially and their relative delay antimesometrially in relation to the elective site of implantation; (3) the induction of asynchrony between the blastocyst and the endometrium, and an attempt to interfere with implantation. BIBLIOGRAPHIC REFERENCES: Davies, J., and L.H. Hoffman 1975 Studies on the progestational endometrium of the rabbit. II. Electron microscopy, day 0 to day 13 of gonadotrophin-induced pseudopregnancy. Am. J. Anat., 142:335-365. Orgebin-Christ, M.-C., Benjamin J. Danzo, and J. Davies 1975 Endocrine control of the development and maintenance of sperm fertilizing ability in the epididymis. In: Handbook of Physiology - Endocrinology V. American Physiological Society, Washington, D.C., pp. 319-338.